AChandraX‐Ray Study of NGC 1068. I. Observations of Extended Emission

Abstract

We report subarcsecond-resolution X-ray imaging spectroscopy of the archetypal type 2 Seyfert galaxy NGC 1068 with the Chandra X-Ray Observatory. The observations reveal the detailed structure and spectra of the 13 kpc extent nebulosity previously imaged at lower resolution with ROSAT. The Chandra image shows a bright, compact source coincident with the brightest radio and optical emission; this source is extended by ≃1.5 (165 pc) in the same direction as the nuclear optical line and radio continuum emission. Bright X-ray emission extends ≃ 5 (550 pc) to the northeast and coincides with the northeast radio lobe and gas in the narrow-line region. The large-scale emission shows trailing spiral arms and other structures. Numerous point sources associated with NGC 1068 are seen. There is a very strong correlation between the X-ray emission and the high-excitation ionized gas seen in Hubble Space Telescope and ground-based [O III] λ5007 images. The X-rays to the northeast of the nucleus are absorbed by only the Galactic column density and thus originate from the near side of the disk of NGC 1068. In contrast, the X-rays to the southwest are more highly absorbed and must come from gas in the disk or on the far side of it. This geometry is similar to that inferred for the narrow-line region and radio lobes. Spectra have been obtained for the nucleus, the bright region ≃ 4 to the northeast, and eight areas in the extended emission. The spectra are inconsistent with hot plasma models, the best approximations requiring implausibly low abundances (≾ 0.1 Z_☉). Models involving two smooth continua (either a bremsstrahlung plus a power law or two bremsstrahlungs) plus emission lines provide excellent descriptions of the spectra. The emission lines cannot be uniquely identified with the present spectral resolution (~110-190 eV), but are consistent with the brighter lines seen in the XMM-Newton reflection grating spectrometer (RGS) spectrum below 2 keV. Hard X-ray (above 2 keV) emission, including an iron line, is seen extending ≃ 20 (2.2 kpc) northeast and southwest of the nucleus. Lower surface brightness, hard X-ray emission, with a tentatively detected iron line, extends 50 (5.5 kpc) to the west and south. Our results, when taken together with the XMM-Newton RGS spectrum, suggest photoionization and fluorescence of gas by radiation from the Seyfert nucleus to several kpc from it. The facts that (1) the large-scale (arcminute) and small-scale (few arcseconds) X-ray emissions align well and (2) the morphology of the large-scale emission does not correlate well with the starburst suggests that the starburst is not the dominant source of the extended X-rays.